Disruption of the cell-cell junction with concomitant changes in the expression of junctional proteins is a hallmark of cancer metastasis and invasion. Role of adherent junction proteins have been studied extensively in cancer, however the role of tight junction proteins is less understood. Claudins are the recently identified tetraspanins, which are integral to the structure and function of tight junctions (TJs). Recent studies have shown changes in expression/cellular localization for claudins during tumorigenesis, however a cause and effect relationship has not been established. Here, we report a highly increased expression for claudin-1 in human primary colon carcinoma and metastatic tissues and cell lines derived from similar sources with relatively frequent nuclear localization. Furthermore, using genetic manipulations of claudin-1 expression in colon cancer cell lines, we demonstrate a role for claudin-1 in the regulation of epithelial to mesenchymal transition (EMT), growth of xenografted tumors and metastasis in athymic mice. We further show regulation of E-cadherin expression and ?-catenin/Tcf signaling as a possible mechanism for claudin-1 dependent EMT. [unreadable] [unreadable]